If I want to play it with a different group, one the people who have it are not part of, then I consider buying it.
I've also bought some games that I just had to have because they were just that awesome. And then never played, because other people already have a copy, and sometimes they have more content or whatever. Terraforming Mars, for example, I ended up only playing it a handful of times solo, and then the app came out, and I haven't opened my copy since. I try to avoid doing this now, because it really doesn't work out that well.
Although I am considering buying Brass: Birmingham so I can decide when I want it to be available in a meetup. Also Spirit Island, but that's because I think I can get my kids to play with me.
It's my favorite series of his. I read the first couple of Laundryverse books, and while they're fun, I'm not a fan of the lovecraftian horror thing. But Merchant Princes hooked me right from the start. Tons of politicking, and by the end it gets messy, like really messy. It's basically The Godfather meets Game of Thrones meets Sliders. And then the followup series (Empire Games) is a Cold War spy thriller with portals. You can just start with Empire games, it's written to be a separate series, but it does have massive spoilers for the original series.